Free-machining Fe-Ni-Co alloy

ABSTRACT

An iron-nickel-cobalt alloy is described which has the following weight percent composition:  
                                           Carbon    0.04 max.         Manganese    0.50 max.         Silicon    0.20 max.         Sulfur   0.020 max.         Cobalt   16-18         Nickel   28-31         Boron   0.020 max.                                    
 
     The alloy also contains 0.01-0.50% of an element selected from the group consisting of bismuth, lead, selenium, and combinations thereof. The balance of the alloy is essentially iron and the usual impurities. The alloy according to this invention provides a unique combination of machinability, low thermal expansion, phase stability, and hot workability.

[0001] This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/308,007 filed Jul. 26, 2001.

FIELD OF THE INVENTION

[0002] This invention relates to Fe—Ni—Co alloys that provide controlled thermal expansion, and in particular, to such an alloy that also provides a unique combination of machinability, processability, thermal expansion, and glass-sealing capability.

BACKGROUND OF THE INVENTION

[0003] An alloy that is sold under the registered trademark KOVAR by Carpenter Technology Corporation is a Fe—Ni—Co alloy that has the following nominal composition, in weight percent. Carbon  0.02% max. Manganese  0.30% Silicon  0.20% Nickel 29.00% Cobalt 17.00% Iron Balance

[0004] The KOVAR alloy provides a low coefficient of thermal expansion (COE) which extends over a wider temperature range than the COE provided by the INVAR alloy (36Ni—Fe). This is primarily because of the presence of cobalt in the KOVAR alloy. The COE of the KOVAR alloy is closely matched with many hard glasses and ceramics such as borosilicate glass, 91-99% opaque alumina, optical fibers, and beryllium oxide.

[0005] The KOVAR alloy is used in applications requiring metal-to-glass and metal-to-ceramic seals. The alloy is also used in a variety of other devices including optical fiber packages, cellular telephone components, frames for the lens and/or light source in compact projectors, frames for laser devices and microwave tubes, and lids for the hermetic sealing of ceramic multilayer semiconductor packages. For many of those parts, more than 90% of the bulk KOVAR material may be removed during machining of the parts.

[0006] The KOVAR alloy has an essentially austenitic structure and is characterized by a strong work-hardening behavior. It has been found that the known commercial grades of the alloy leave something to be desired when used in large scale machining operations because of the strong work-hardening characteristic. The machining chips from the known grades of the KOVAR alloy are gummy and not easily broken. This results in undesirable wear of the machining tool. It has also been found that workpieces of the KOVAR alloy will deflect during machining because of the higher cutting force used to obtain an acceptable production rate. As a consequence of these problems, precision parts with close dimensional tolerances have not been easily machined from the known grades of the KOVAR alloy.

[0007] Attempts have been made to improve the machinability of the KOVAR alloy by including a small amount of sulfur in the alloy. In practice, however, it was found that more than about 0.015% sulfur adversely affects the hot workability of the alloy. When sulfur is restricted to less than 0.015% in the KOVAR alloy, the benefit to machinability was found to be marginal.

SUMMARY OF THE INVENTION

[0008] The need for a true free-machining Fe—Ni—Co alloy such as the KOVAR alloy is solved to a substantial degree by the alloy according to the present invention. The alloy of this invention is a low thermal expansion Fe—Ni—Co alloy that contains a small addition of bismuth to benefit the machinability of the alloy. The alloy according to this invention has the following broad and preferred compositions in weight percent. Broad Preferred Carbon  0.04 max. 0.010 max. Manganese  0.50 max. 0.35-0.45 Silicon  0.20 max. 0.08-0.15 Sulfur 0.020 max. 0.004 max. Cobalt 16-18 16.8-17.75 Nickel 28-31 28.8-29.6 Bismuth + Lead + Selenium 0.01-0.50 0.08-0.25 Boron 0.020 max. 0.018 max.

[0009] The balance of the alloy is essentially iron and the usual impurities found in commercial grades of Fe—Ni—Co alloys intended for similar service or use.

[0010] The foregoing tabulation is provided as a convenient summary and is not intended to restrict the lower and upper values of the ranges of the individual elements of the alloy of this invention for use in combination with each other, or to restrict the ranges of the elements for use solely in combination with each other. Thus, one or more of the element ranges of the broad composition can be used with one or more of the other ranges for the remaining elements in the preferred composition. In addition, a minimum or maximum for an element of one preferred embodiment can be used with the maximum or minimum for that element from another preferred embodiment. Throughout this application, the term “percent” or the symbol “%” means percent by weight, unless otherwise indicated.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0011] The foregoing summary as well as the following detailed description of a preferred embodiment of the present invention will be better understood when read with the appended drawings, wherein:

[0012]FIG. 1 is a photograph of the machining chips obtained from Example Alloys A, B, and 1 to 6, of the Working Examples;

[0013]FIG. 2 is a photomicrograph of a sample of Example Alloy 1;

[0014]FIG. 3 is a graph of the energy dispersion spectrum of the sample shown in FIG. 2; and

[0015]FIG. 4 is a photograph of glass/metal sealing test assemblies utilizing a metal substrate formed from Example Alloy 7 of the present invention.

DETAILED DESCRIPTION

[0016] Up to about 0.50% manganese is present in the alloy of this invention primarily as a deoxidizer. Manganese also benefits the austenite stability in this alloy and enables the alloy to resist martensitic transformation, even at relatively low temperatures. Manganese contributes to the superior machinability provided by this alloy by combining with available sulfur to form manganese sulfides. Preferably, the alloy contains at least about 0.35% manganese. However the presence of too many manganese sulfides, especially in the grain boundaries, adversely affects the processability of the alloy, particularly its hot-workability. Too much manganese also adversely affects the COE of this alloy. For these reasons, manganese is preferably limited to not more than about 0.45%.

[0017] Up to about 0.04% carbon is also present in the alloy as a deoxidizer. Carbon also benefits the austenite stability in this alloy. However, carbon is preferably limited to not more than about 0.010% because too much carbon adversely affects the COE of this alloy.

[0018] Up to about 0.20% silicon is present in this alloy from deoxidizing additions during melting and refining of the alloy. Preferably the alloy contains at least about 0.08% silicon. Too much silicon adversely affects the phase stability of the alloy. Accordingly, silicon is preferably limited to not more than about 0.15%.

[0019] This alloy contains at least about 28%, preferably at least about 28.8%, nickel because nickel contributes to the low COE provided by this alloy. Although nickel benefits the stability of the austenitic structure, too much nickel results in a higher COE which may be undesirable for some uses. Therefore, nickel is limited to not more than about 31% and preferably to not more than about 29.6% in this alloy. For best results, the alloy contains about 29.4% nickel.

[0020] Cobalt, like nickel, contributes to the low COE of this alloy. Cobalt also extends the alloy's useful temperature range because it increases the Curie temperature (T_(c)) of the alloy. Accordingly, the alloy contains at least about 16% and preferably at least about 16.8% cobalt. Too much cobalt results in the COE becoming too high for certain uses. Therefore, cobalt is restricted to not more than about 18%, and preferably to not more than about 17.75% in this alloy. For best results, the alloy contains about 17.5% cobalt.

[0021] A small amount of sulfur, up to about 0.020%, preferably up to about 0.015%, may be present in this alloy to combine with manganese and form manganese-sulfides that benefit the machinability of the alloy. Toward that end, the alloy contains at least about 0.008% sulfur, and preferably at least about 0.010% sulfur. Sulfur adversely affects the hot workability of the alloy and an excessive amount will cause hot cracking of the alloy during forging or hot rolling. Therefore, when optimum hot workability is needed, sulfur is restricted to not more than about 0.004%, and preferably to not more than about 0.003% in this alloy.

[0022] A small but effective amount of bismuth, up to about 0.50%, preferably up to about 0.25%, is present in this alloy to benefit the machinability property, particularly in turning and form tool machining operations. Toward that end the alloy preferably contains at least about 0.01%, and better yet, at least about 0.08% bismuth. For best results, the alloy contains about 0.10 to 0.20% bismuth.

[0023] The bismuth is present in the alloy as a dispersion of fine particles that are typically about 1 μm in major dimension. The advantage of including bismuth instead of sulfur to benefit machinability is that the insoluble bismuth particles are in the form of fine, isotropically shaped inclusions which do not concentrate at the grain boundaries. Therefore, they do not embrittle the alloy even if they melt at the hot working temperature. This phenomenon is in contrast to manganese sulfide inclusions which are mostly distributed in elongated “stringers” along the grain boundaries in high nickel alloys such as the KOVAR alloy. At forging and hot rolling temperatures, the manganese sulfides melt and thereby weaken the grain boundaries. This process leads to hot cracking as the alloy is hot worked. The bismuth particles have a relatively low melting point and act as a lubricant for the cutting tool when the alloy is subjected to high speed machining operations. This not only significantly increases the cutting tool life, but also alleviates the need to use a greater amount of force on the cutting tool. Most importantly, the presence of bismuth in this alloy does not adversely affect the other desirable properties of the KOVAR alloy such as the low COE and the austenite stability.

[0024] A small amount of lead, up to about 0.50%, but preferably not more than about 0.25%, may be present in this alloy to benefit the machinability of the alloy for certain uses where its toxicity is not a concern. To realize the benefit to machinability, the alloy contains at least about 0.01% and better yet, at least about 0.08% lead However, because of its toxicity, lead is preferably not used in this alloy and is preferably restricted to not more than about 0.01% and better yet to not more than about 0.005% in the alloy.

[0025] A small amount of selenium up to about 0.50%, preferably up to about 0.25%, may also be present in this alloy to benefit the machinability property. Toward that end, the alloy contains at least about 0.01% and better yet, at least about 0.08% selenium. However, selenium is not preferred because if too much is present, the alloy becomes susceptible to hot cracking during forging or hot rolling. Accordingly, where resistance to hot cracking is important, selenium is preferably restricted to not more than about 0.01%, and better yet to not more than about 0.005% in this alloy.

[0026] Bismuth, lead, and selenium may be present individually or in combination in this alloy. Thus, the alloy may contain about 0.01-0.50%, preferably about 0.08-0.25%, of bismuth, lead, and selenium. As noted above, bismuth is preferred for enhancing the free-machining capability of this alloy and when bismuth is used as the free-machining additive in the alloy, lead and selenium are each restricted to not more than about 0.01%, and better yet to not more than about 0.005% in this alloy. Likewise, when lead is used as the free-machining additive in the alloy, bismuth and selenium may each be restricted to not more than about 0.01%, and better yet to not more than about 0.005% in this alloy. Moreover, when selenium is used as the free-machining additive in the alloy, lead and bismuth may each be restricted to not more than about 0.01%, and better yet to not more than about 0.005% in this alloy.

[0027] A small amount of boron up to about 0.020%, preferably up to about 0.018% may be present in this alloy to benefit the hot-workability of the alloy.

[0028] The balance of the alloy is iron and the usual impurities found in commercial grades of Fe—Ni and Fe—Ni—Co alloys intended for similar use and service.

[0029] No special techniques are needed to make the alloy of this invention. The alloy is preferably vacuum induction melted and cast into ingot form. Vacuum melting is preferred because it provides a strong stirring action to the molten alloy which results in a substantially homogenous distribution of the bismuth particles when the alloy is consolidated. The alloy is readily hot worked and/or cold worked to a desired shape and cross-sectional dimension. The alloy is preferably hot worked from a temperature of about 1900° F., and better yet from a temperature of about 1850-1900° F. Billets of the alloy are preferably ground and polished prior to hot rolling to minimize edge cracking. The Fe—Ni—Co alloy according to this invention can be processed into forms such as bar, plate, wire, rod, and strip. The alloy can be readily machined into precision parts for glass-to-metal and ceramic-to-metal seals in electron tubes, integrated circuits, and other electronic devices. The alloy is also useful for optical fiber packages, cellular telephone components, frames for the lens and/or light source in compact projectors, frames for laser devices and microwave tubes, lids for the hermetic sealing of ceramic multilayer semiconductor packages, and other devices.

[0030] The alloy according to the present invention provides a coefficient of thermal expansion as follows, when measured according to the method described in ASTM E228: Temp. COE 25-400° C. 4.6-5.5 × 10⁻⁶/° C. 25-450° C. 4.9-5.7 × 10⁻⁶/° C.

[0031] The inflection point of the alloy is about 400-450° C. The alloy has an austenitic structure at room temperature, some of which may transform to martensite at very low temperatures (e.g., −80° C. or lower). The stability of the austenitic phase is affected by the composition and processing of the alloy. When used in a product form having a cross-sectional area greater than strip, e.g., bar or rod, the composition may be less homogeneous. Nevertheless, the alloy is not expected to provide significant transformation to martensite when tested by observing the cross section of a test specimen on which the smallest dimension is larger than 0.5″, after the test piece is deep chilled at −80° C. for at least 4 hours. Therefore, the free-machining Fe—Ni—Co alloy of this invention may be used in applications where larger cross-sectional parts (e.g., ≧0.5 inch in smallest dimension) are needed.

[0032] The Fe—Ni—Co alloy of this invention provides superior machinability compared with the known grades of the KOVAR alloy. For heavy gauge strip products made with this alloy, it is also expected that the alloy will provide much improved stamping characteristics.

Working Examples

[0033] Eleven (11) experimental heats were made by vacuum induction melting (VIM) and cast into ingots. Alloys 1-6, A, and B were prepared as small (32 lb.) heats. Alloys C and D were obtained from samples of commercially available Fe—Ni—Co alloys. Alloy 7 was prepared as a 400 lb. heat. The chemical analyses of the experimental alloys are listed in Table I below. All values are in weight percent. For the Bi-bearing heats, bismuth pellets (3-8 mm diam.) were added to the melt before tapping. The ingots were hot-forged to about 0.5-0.75″ thick. TABLE I Alloy ID 1 2 3 4 5 6 7 A B C D C 0.002 0.001 0.001 0.001 0.001 0.001 0.002 0.001 0.001 0.005 0.005 Mn 0.4 0.39 0.38 0.36 0.39 0.38 0.4 0.24 0.24 0.25 0.43 Si 0.1 0.08 0.09 0.08 0.09 0.08 0.1 0.1 0.1 0.12 0.11 P 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.002 S 0.015 0.001 0.001 0.007 0.001 0.001 0.001 0.001 0.01 0.01 0.002 Cr 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.07 Ni 29.2 29.46 30.05 29.6 29.62 29.37 29.41 29.05 29.13 29.85 30.1 Mo 0.01 0.1 0.1 0.1 0.1 0.1 0.01 0.01 0.01 0.02 0.02 Cu 0.01 0.09 0.09 0.09 0.09 0.07 0.01 0.01 0.01 0.05 0.04 Co 17.52 17.71 17.02 17.53 17.42 17.44 17.49 17.48 17.53 17.02 17.82 Ti 0.04 0.03 0.03 0.03 0.03 0.03 0.02 0.01 0.01 0.01 0.024 Al 0.02 0.02 0.01 0.03 0.03 0.03 0.01 0.01 0.01 0.01 0.082 Zr 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.008 B 0.013 0.01 0.01 0.007 0.006 0.007 0.012 0.006 Bi 0.08 0.25 0.23 0.084 0.14 Mg 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0005 0.0006 0.002 W 0.03 0.03 0.03 0.03 0.01 0.03 0.03 0.01 V 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Pb 0.15 Se 0.33

[0034] Heats 1-7 are representative embodiments of the alloy according to the present invention. Heat A is an example of the known grade of the KOVAR alloy. Heat B is essentially the same as Heat A, but contains a small addition of sulfur. Heats C and D are comparative examples of other commercially available grades of Fe—Ni—Co alloys.

[0035] Set forth in Table II are qualitative ratings of the forgeability of Alloys A, B, and 1 to 7. All ingots were pre-heated at 2100° F. for 2 h prior to being forged. Also shown are qualitative ratings of the hot rolling of Alloy 7 when hot rolled from preheat temperatures of 2100° F., 2000° F., and 1900° F., respectively. TABLE II Pre-heat Forged Hot rolled temp./time 2100° F./2 h 2100° F./2 h 2000° F./2 h 1900° F./2 h Alloy ID A Smooth surface B Very little edge cracking 1 Some edge cracking 2 Very little edge cracking 3 Very little edge cracking 4 Some edge cracking 5 Some edge cracking 6 Significant edge cracking 7 Very little edge Significant Severe edge Very little cracking edge cracking cracking edge cracking

[0036] Alloys 2 and 3 are preferred compositions and showed good hot-workability. Therefore, it is practical to produce a bismuth-bearing Fe—Ni—Co alloy according to this invention without incurring significant yield loss during forging or hot-rolling. Alloys 1 and 4 show the adverse effect of sulfur on the hot workability. Alloy 5, which contains lead, provided acceptable forgeability. The results for Alloy 6 show the adverse effect of too much selenium on the hot workability of this alloy.

[0037] Samples for COE and transformation tests were cut from the forged billets of each heat. The results of the thermal expansion and transformation testing are shown in Table III. TABLE III Alloy 1 2 3 4 5 6 7 A B C Transformation* 3% 0% 0% 0% 0% 0% 0% 5% 1% 0% COE(30-400° C.) 4.75, 4.96 5.34, 5.43 5.41 5.45 5.49 5.47 5.15 4.62 4.76 5.35, 5.09 COE(30-450° C.) 5.17, 5.20 5.57, 5.58 5.44 5.61 5.69 5.57 5.43 4.93 5.02 5.50, 5.30

[0038] The data in Table III show that Alloys 1-7 provide very good phase stability compared to Alloys A and B. The data also show that Alloys 1-7 provide acceptable thermal expansion over the temperature range of interest. By way of comparison, ASTM F-15, Standard Specification for Iron-Nickel-Cobalt Sealing Alloy, specifies the following COE's: (30-400° C.) 4.6-5.2 ppm/° C., (30-450° C.) 5.1-5.5 ppm/° C.

[0039] Machining test specimens were prepared from the forged billets of Alloys 1-6, A, and B. The machinability of each alloy was evaluated using a single-point turning test. The specimens were turned on an RTW lathe at 415 RPM, at a feed rate of 0.004 inches per revolution (ipr), and a 0.100 inch depth of cut. The depth of cut for the specimen of Alloy A was reduced to 0.050 inch to prevent bending of the specimen. Set forth in Table IV are the qualitative results of the machining test. TABLE IV Alloy ID Machinability A Very gummy with very poor chips which could not be easily broken. Very poor roughing finish. B Gummy with thick chips that were hard to break or curl. Very poor roughing finish. Material was very soft. 1 Machined better than Alloys A and B. Produced “6” and “9” shaped chips. Rough finish not that good, but did not have to reduce the depth of cut to keep sample from bending. 2 Cuts very easily. Very good chips with “6” and “9” shapes. No problem machining from square to round. Typical surface finish for roughing cuts that are easy to machine. Surface finish has some marks. Very minimal wear observed on cutting tool inserts after rough cut. 3 Material cuts easily. On the first pass in the turning round sample, very good chips with “6” and “9” shapes. Next cutting at smaller diameter at same feed rate and cutting speed, good chip breaks after two coils. Surface finish was good with very few cut marks. Minimal wear observed on cutting tool inserts after rough cut. 4 Good chips during first pass. Second pass at 0.100 in. cut depth resulted in poorer chips. The material was gummy and did not curl into “6” and “9” shapes. Chips pushed up onto the cutting tool inserts before breaking off. In the third pass (0.050 in. cut depth) resulted similar type chips. The next pass provided some “6” and “9” shapes. However, the chips became gummy after about one inch. Tearing of material observed as opposed to cutting. The last pass (0.030 in. cut depth) resulted in more wear on the cutting tool insert. 5 Chips did not break in “6” and “9” shapes. They came off thickly before breaking. When 0.050 in. cut depth was used “6” and “9” shapes were initially obtained, but then turned to thick flat chips with little or no curl. The material was gummy. In the next to last pass some chips had “6” and “9” shapes. The last pass produced long chips which gave good surface finish. Some wear observed on the cutting tool insert. 6 The first and second passes provided acceptable chips that broke easily. With the 0.050 in. cut depth, good chip curling was observed with some “6” and “9” shapes.

[0040]FIG. 1 shows the machining chips obtained from each of the comparative alloys and the alloys of the present invention.

[0041] The example of the known grade of the KOVAR Alloy (Alloy A) was difficult to machine and provided long, gummy chips, which are undesirable. Significant tool wear and metal bending occurred in machining Alloy A. In contrast, Alloy B and Alloys 1-6 all showed improved machinabilities, at different degrees, with comparatively easy-breaking chips. Alloy B is an example of a resulfurized grade of the KOVAR alloy. However, it is marginally better than Alloy A with some gummy chips. Alloys 2 and 3 with only a bismuth addition showed the best machining performance.

[0042]FIG. 2 shows the microstructure of Example Alloy 1 (0.015% S and 0.08% Bi). Energy dispersion spectrum (EDS) analysis of Alloy 1, shown in FIG. 3 indicates there are two types of inclusions present. One is the elongated manganese sulfide (Mn(Ti)S) which is typically about 5 μm or longer. The other is the bismuth particle which is isotropic and typically about 1 μm in diameter. Therefore, in the alloy according to this invention, there is the option to introduce one or both types of inclusions by adjusting the amounts of manganese, sulfur, and bismuth.

[0043] To demonstrate the glass sealing capability of the alloy according to this invention, a test was performed with specimens machined from the billet material of Alloy 7. In this test, test pieces, each 1 in.×3 in.×0.25 in., were cut from the billet material of Alloy 7. A flat surface of each piece was polished and then cleaned with acetone. A piece of double strength window glass strip was cleaned with acetone and then placed on the polished surface of each of the metal pieces. The metal/glass assemblies for this example are shown in FIG. 4.

[0044] The metal/glass assemblies were heated in air at 2025° F. for 6 minutes, and then cooled to room temperature. The metal/glass assemblies were visually inspected for the presence of bubbles trapped in the glass as a result of melting and resolidifying. The sealing capability of the metal/glass assemblies is assessed based on a scale of 1 to 5, wherein a rating of 1 indicates no bubbles observed and a rating of 5 indicates a large number of bubbles observed. The assemblies shown in FIG. 4 were rated 1-2, indicating acceptable glass-sealing performance.

[0045] A novel glass-sealing alloy has been described. This alloy provides significantly better machinability than the known grades of low thermal expansion, iron-nickel-cobalt glass-sealing alloys, such as the KOVAR alloy. The alloy according to this invention also provides acceptable levels of hot workability, thermal expansion, and glass-sealing capability. Thus, the alloy according to the present invention provides a unique combination of properties relative to the closest known grades of iron-nickel-cobalt alloys intended for similar use or service.

[0046] The terms and expressions that have been employed herein are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions to exclude any equivalents of the features described or any portions thereof. It is recognized, however, that various modifications are possible within the scope of the invention described and claimed herein. 

What is claimed is:
 1. A controlled thermal expansion alloy consisting essentially of, in weight percent, about: Carbon  0.04 max. Manganese  0.50 max. Silicon  0.20 max. Sulfur 0.020 max. Cobalt 16-18 Nickel 28-31 Boron 0.020 max.

0.01-0.50% of an element selected from the group consisting of bismuth, lead, selenium, and combinations thereof, and the balance of the alloy being essentially iron and the usual impurities.
 2. An alloy as set forth in claim 1 which contains not more than about 0.01% lead and not more than about 0.01% selenium.
 3. An alloy as set forth in claim 2 which contains at least about 0.08% bismuth.
 4. An alloy as set forth in claim 3 which contains not more than about 0.25% bismuth.
 5. An alloy as set forth in claim 1 which contains at least about 0.35% manganese.
 6. An alloy as set forth in claim 5 which contains at least about 0.008% sulfur.
 7. An alloy as set forth in claim 1 which contains not more than about 0.004% sulfur.
 8. An alloy as set forth in claim 7 which contains not more than about 0.01% lead and not more than about 0.01% selenium.
 9. An alloy as set forth in any of claims 1 to 8 which contains not more than about 0.25% bismuth.
 10. An alloy as set forth in claim 9 which contains at least about 0.08% bismuth.
 11. A controlled thermal expansion alloy consisting essentially of, in weight percent, about: Carbon  0.01 max. Manganese  0.45 max. Silicon  0.15 max. Sulfur 0.015 max. Cobalt 16.8-17.75 Nickel 28.8-29.6 Boron 0.018 max.

0.08-0.25% of an element selected from the group consisting of bismuth, lead, selenium, and combinations thereof, and the balance of the alloy being essentially iron and the usual impurities.
 12. An alloy as set forth in claim 11 contains not more than about 0.01% lead and not more than about 0.01% selenium.
 13. An alloy as set forth in claim 12 which contains at least about 0.08% bismuth.
 14. An alloy as set forth in claim 13 which contains not more than about 0.25% bismuth.
 15. An alloy as set forth in claim 11 which contains at least about 0.35% manganese.
 16. An alloy as set forth in claim 15 which contains at least about 0.008% sulfur.
 17. An alloy as set forth in claim 11 which contains not more than about 0.004% sulfur.
 18. An alloy as set forth in claim 17 which contains not more than about 0.01% lead and not more than about 0.01% selenium.
 19. An alloy as set forth in any of claims 11-18 which contains at least about 0.10% bismuth.
 20. A controlled thermal expansion alloy consisting essentially of, in weight percent, about: Carbon 0.01 max. Manganese 0.35-0.45 max. Silicon 0.08-0.15 max. Sulfur 0.004 max. Cobalt 16.8-17.75 Nickel 28.8-29.6 Bismuth 0.10-0.20 Boron 0.018 max.

and the balance is essentially iron and the usual impurities. 